1. Field of the Invention
The invention relates to a process for producing a polyimide, and more particularly to a process for producing a polyimide that can produce a polyimide which is soluble in a solvent and has high processability and stability.
2. Prior Art
Polyimides have been extensively applied as heat-resistant resins having excellent mechanical properties, electric properties, chemical resistance, weathering resistance and other properties in the field of electric and electronic industries. Polyimides, however, have low solubility in an solvent and thus cannot be prepared as a polyimide resin solution for casting or impregnation. For this reason, polyimides are produced by dissolving a polyamic acid, having high solubility in a solvent, as a precursor of a polyimide in an organic polar solvent to prepare a resin solution, processing the resin solution, and then conducting heat treatment and chemical treatment to cause a dehydration ring-closing reaction (Lee, Stoll, and Nevills, xe2x80x9cAtarashii Tainetsusei Jushi (New Heat-resistant Resin),xe2x80x9d Tokyo Kagaku Dozin Co., Ltd., p. 216).
The two-stage process, wherein processing is carried out in the state of polyamic acid followed by imidation, can compensate for low processability of the polyimide. In this process, however, since processing should be carried out in the state of polyamic acid, which is easily hydrolyzed in the presence of water, that is, has low stability, it is very difficult to stably provide high-quality product.
Accordingly, it is an object of the invention to provide a process for producing a polyimide that can produce a polyimide which is soluble in a solvent and has high processability and stability.
According to the first feature of the invention, a process for producing a polyimide having an average molecular weight of 10,000 to 300,000, comprises the step of heating an acid dianhydride and a diamine in an organic polar solvent in the presence of xcex3-caprolactone or xcex2-butyrolactone as an acid catalyst.
The above diamine is preferably one or two or more compounds selected from the group consisting of 
The acid dianhydride is preferably one or two or more compounds selected from the group consisting of 
In the invention, when a single diamine is reacted with a single acid dianhydride, a homopolymer is produced, while, when two or more diamines or two or more acid dianhydrides are used, a random or block copolymer is produced.
The use of xcex3-caprolactone or xcex2-butyrolactone as an acid catalyst in an organic polar solvent can cause ring closing of polyamic acid comprising an acid dianhydride and a diamine and thus can produce a copolymerizable high-molecular weight polyimide in one stage.
Organic polar solvents usable herein include, for example, N,N-dimethylformamide, N,N-dimethylacetamide, dimethylsulfoxide, N-methyl-2-pyrrolidone, xcex3-butyrolactone, and 1,3-dimethyl-2-imidazolidinone.